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Measuring LED and Solid State Lighting Performance

Thursday, 01 January 2009

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Light emitting diodes (LEDs) and solid state lighting (SSL) products that incorporate LEDs pose many measurement challenges compared with other lighting elements, such as traditional tungsten and fluorescent. Advanced optical radiation measurement equipment and new techniques are often required to determine basic photo-metric and colorimetric parameters for LEDs and SSL products. Updated measurement instrumentation, calibration and performance characterization methods have allowed for improvement in the repeatability and reproducibility of measurements of average LED intensity, total luminous flux and colorimetric quantities. Lower uncertainty, detector-based standards provide a convenient transfer and monitoring from primary measurement standards.

High sensitivity spectroradiometers using backside-thinned charge-coupled device (CCD) detectors provide much improved sensitivity in the blue spectral region over previous array-based systems. This is very important for obtaining consistent photometric performance results from white SSL products. New methods of characterizing the integrated stray-light performance of these detector array-based spectroradiometers allows the determination of colorimetric quantities from narrow spectral emission LEDs to almost match the results obtained from double-grating, scanning spectroradiometers. Because LEDs and SSL products can have significant variation in color as a function of direction of the emitted light, measurements as a function of angle or gonio-metric characterization is required, further complicating the quality assessment. The use of high quality measurement instrumentation is a basic requirement but this alone does not assure that consistent and repeatable results will be obtained.

Why the large recent increase in attention to LEDs and solid state lighting? The cost of energy and the amount of carbon emissions is one answer. Lighting consumes 22% of the electricity produced in the US and 8% of the energy. The impact of SSL and other alternative lighting sources by 2025 is projected to be a 50% decrease of energy consumption by lighting and a 10% reduction in carbon emissions.1 The Department of Energy and many manufacturers are very interested in increasing the efficiency conversion of electrical power to light output. This is the key performance factor that, depending on the application, determines the level of energy-efficiency.

For applications where LEDs are used as sources of light that are viewed directly, the term luminance intensity is used (i.e. units of lumen/steradian, also known as candela). For example, in a traffic intersection signal light, the important parameter is the light emitted into a solid cone angle. For applications where LEDs are used as general illumination, the total light emitted in all directions, the total luminous flux (units of lumen), is the metric of interest. The efficiency of the conversion of electrical
power into light has a specific term, luminous efficacy, and a defined relationship of measurable parameters: 1) optical power in watts, 2) electrical power in watts, and 3)luminous flux in lumens.2

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